1. Field of the Invention
The present invention relates in general to electric motors and more particularly to brushless motors suitable for driving a blower used in an automotive air conditioning device.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional brushless motor used for the above-mentioned field will be briefly described.
The brushless motor comprises a driving unit, a drive shaft powered by the driving unit and a parts protecting case. The parts protecting case houses therein the driving unit. The drive shaft projects upward from the case. A blower of the air conditioning device is fixed to the drive shaft. The driving unit comprises a yoke fixed to the drive shaft, a plurality of magnets circumferentially arranged and secured to an inner surface of the yoke having S- and N-poles thereof arranged alternately, a fixed housing for rotatably supporting the drive shaft and a stator mounted to the fixed housing. Upon energization of the driving unit, the yoke and thus the drive shaft is rotated at a high speed relative to the fixed housing.
However, in the brushless motor having the abovementioned structure, it is very difficult to smoothly rotate the drive shaft because of inevitable vibration of the yoke. If, due to the vibration of the yoke, the drive shaft fails to rotate smoothly, the parts protecting case tends to produce a marked noise caused by resonance.